。
She practices piano day and night.
Literal
She [topic-は] daytime also night also piano [object-を] is-practicing.
昼も夜も ('both day and night') uses repeated も to emphasize the all-encompassing scope of her practice. ピアノを練習している ('is practicing piano') uses the progressive ~ている for an ongoing activity. The 昼も夜も pattern expresses relentless dedication or obsession with an activity.